1. Field of the Invention
The present invention relates to an infrared sensor and a manufacturing method thereof and, in particular, to a pixel construction of an uncooled infrared sensor and a manufacturing method thereof.
2. Related Art
Infrared image sensing is characterized in its ability to pick up images at night as well as during the daytime and its higher transmittance to smoke and fog than visible radiation. Being capable of acquiring temperature information of an object to be sensed, infrared image sensors are adaptable to a variety of applications, such as a monitor camera and a fire detection camera, besides the field of defense.
In recent years, extensive studies have been made on “uncooled infrared solid-state imaging elements” which eliminate the need for the cooling mechanism to allow for a low-temperature operation, which is the most critical shortcoming of a quantum type infrared solid-state imaging element or a leading conventional type. In such an uncooled infrared solid-state imaging device, an incident infrared ray having a wavelength of 10 μm or so is converted into heat by an absorber and then the temperature change caused by this small amount of heat at a heat sensing portion is converted into an electric signal by thermoelectric conversion means of any kind, before reading out the electric signal to obtain infrared image information.
There are three ways of improving the sensitivity of such an uncooled infrared sensor, generally classified as follows:
The first method is to improve the ratio of an infrared power incident to an infrared detection portion, dP, to a temperature change of an object, dTs, namely dP/dTs. This method achieves improvement mainly by optical systems, which corresponds to enlargement of an infrared lens aperture, application of an antireflective coating, use of a lens material of low optical absorption, increase of an infrared absorptivity at an infrared detection portion or increase of an infrared absorption area. In accordance with the recent multiplication of pixels for an uncooled infrared sensor, the size of a unit pixel has mainly come to as small as 40 μm×40 μm and, among the items mentioned above, the increase of an infrared absorption area at an infrared detection portion has remained a relatively critical problem to be solved. However, a report has been published that an infrared absorption area can be increased up to 90% of a pixel area by stacking an infrared absorption layer on top of the pixel (Tomohiro Ishikawa, et al., Proc. SPIE Vol. 3698, p. 556, 1999) and it is difficult to obtain a further considerable amount of improvement in sensitivity by any optical means.
The second method is to improve the ratio of a temperature change at an infrared detection portion, dTd, to an incident infrared power, dP, namely dTd/dP. Whereas the aforementioned method is an optical one, this can be said to be thermal. Generally speaking, in an uncooled infrared sensor to be packed in a vacuum package, transportation of heat from an infrared detection portion to a supporting substrate is predominantly accomplished by heat conduction by a supporting structure which supports the infrared detection portion in a cavity within the supporting substrate. Thus, attempts have been made to lay a leg-like supporting structure made of a material of low thermal conductivity as narrowly and longways as possible within the constraint of the design. See Tomohiro Ishikawa, et al., Proc. SPIE Vol. 3698, p. 556, 1999, for example. As shown in FIGS. 14(a) and 14B, a construction has been made such that a pair of narrow, spiral slits 201 is formed around a pixel 200 to form a cavity 204 at the bottom of the element. These spiral channels 202 are used as a supporting structure to support the pixel 200 afloat and wirings 203 are provided to connect to a peripheral circuit. When a pixel is being miniaturized to as small as 40 μm×40 μm or so, however, since fine processing has already been made at the silicon LSI processing level, a considerable degree of further improvement in sensitivity may hardly be realized through refinement of the layout of the supporting structure. Similarly, it is difficult to further reduce the thermal conductivity which is a material characteristic of the supporting structure. In particular, with regard to the wiring for outputting electric signals from the infrared detection portion, it is difficult to realize a considerable amount of improvement in sensitivity in terms of material since there is a requirement contradictory between the electric conduction and heat conduction whose mechanisms are similar.
The third method is to improve the ratio of an electric signal change caused by thermoelectric conversion means, dS, to a temperature change at an infrared detection portion, dTd, namely dS/dTd and is, therefore, electrical in nature. This method is, unlike the other two, is directed to sheer sensitization, that is, improvement of dS/dTd; however, it is quite important to reduce various electric noises generated. Thus, various kinds of thermoelectric conversion means have so far been investigated. Reported are, for example, a thermopile for converting a temperature change into a potential change by means of Seebeck effect (Toshio Kanno, et al., Proc. SPIE Vol. 2269, pp. 450-459, 1994), a bolometer for converting a temperature change into a resistance change by means of the temperature change of a resistor (A. Wood, Proc. IEDM, pp. 175-177, 1993), a pyroelectric element for converting a temperature change into an electric charge by pyroelectric effect (Charles Hanson, et al., Proc. SPIE Vol. 2020, pp. 330-339, 1993), a silicon pn junction for converting a temperature change into a voltage change by certain forward currents (Tomohiro Ishikawa, et al., Proc. SPIE Vol. 3698, p. 556, 1999), etc.
If a comparison is made among those methods, however, no one method is then decisively superior to the others in comprehensive view of their thermoelectric conversion characteristics, noise characteristics and manufacturing methods. For instance, the bolometer is superior in terms of temperature resolution while the silicon pn junction, which can be manufactured by the silicon LSI processing alone, is superior in terms of manufacturing method.
As described above, one of the methods for sensitizing an uncooled infrared sensor is a secondly mentioned thermal one, which improves the ratio of a temperature change at an infrared detection portion, dTd, to an incident infrared power, dP, namely dTd/dP. Generally speaking, transportation of heat from an infrared detection portion to a supporting substrate is predominantly accomplished by heat conduction through a supporting structure which supports the infrared detection portion in a cavity within the supporting substrate. Thus, attempts have been made to lay a leg-like supporting structure made of a material of low thermal conductivity as narrowly and longways as possible within the constraint of the design. When a pixel is being miniaturized to as small as 40 μm×40 μm or so, however, since fine processing has already been made at the silicon LSI processing level, a considerable degree of further improvement in sensitivity may hardly be realized through refinement of the layout of the supporting structure.
Furthermore, with a trend of miniaturizing pixels and supporting structures involved in the development of fine processing technology in silicon LSI processing, the influence of the heat transportation by emission from the bottom of a pixel and supporting structure will predictably be appreciable and the sensitization only through the reduction of heat conduction by the miniaturization of a supporting structure will predictably be restricted by a limit in sensitivity contributable to such emission.
In addition, in order to read signals out from an infrared detection portion, a supporting beam line to be formed within a supporting structure must be provided with a unique wiring layer made of a low thermal conductivity material. For example, a supporting beam line structure using titanium material is known to be effective.
Except for the case where titanium itself is used as a thermoelectric conversion material, however, in addition to the wiring process at an element peripheral circuit, another process will be required for forming a supporting beam line structure alone.